This invention relates to flow measurement and monitoring devices and in particular to optical flow monitoring devices.
Accurate measurement and monitoring of fluid flow is important in many situations. One important application of fluid flow monitoring devices is the monitoring of respirator gas flow. Respiratory circuits are typically composed of flexible tubing with an inside diameter of 15 mm. The flow is bi-directional and peaks as high as 20 liters per minute. The gas mixture in the circuits typically contains N2, O2, CO2, N2O, ethanol, anesthetic and other drugs in varying concentrations. The concentration of all gases including O2 varies from the inspired part of the cycle to the expired portion of the cycle. However, at least 21% O2 is always present in the gas mixture in the circuit, and it generally has much higher concentrations of O2. The CO2 concentration is approximately zero on the inspired part of the cycle and as high as 10% on the expired portion of the cycle. Other gases may or may not be present in varying concentrations. Existing flow measuring products on the market include hot wire anemometer, fine mesh net, and pressure drop sensors. All of these products have as a principal shortcoming that they position an obstruction to the flow that creates a pressure drop in the flow channel. Cleaning of these devices is difficult. Ultrasonic anemometers are also known. Their principal shortcomings are that there are sensitive to gas composition and contaminations. Also, they are difficult to clean because they do not allow the use of a disposable or reusable flow measurement cuvette. Finally, they create pulsed pressure waves in the flow channel, and therefore cannot be placed close to the patient. Optical devices for measuring fluid flow are known. These include laser Doppler anemometers. These devices are expensive and they require seeding the flow with calibrated particles. In addition, they position obstruction in the flow channel.
What is needed is a very reliable and accurate, non-invasive, gas-independent, easy to clean, low cost and portable fluid flow measuring and monitoring device, which can be placed close to the patient.
The present invention provides an optical flow monitor. Fluid flow is determined by correlating two interference signals produced by coherent laser beams passing through a flowing fluid at two spaced-apart paths. The distance between the two paths is known and the correlation of the two signals is used to determine the time required for the fluid to flow between the two paths. In a preferred embodiment actually built and tested by Applicant the correlation is made by having an operator monitor on an oscilloscope the intensities of interference fringes corresponding to each of the two beam paths. Intensity variations in the interference fringes are caused by the same turbulent eddies passing each of the two paths. These turbulent eddies cause fluctuations in the index of refraction of the fluid which produce similar patterns on the oscilloscope which are separated on the oscilloscope time scale by an amount corresponding to the distance between the two beam paths and the flow rate of the fluid. The operator can determine the time difference between the similar patterns in the two beams and knowing the actual distance between the beams the operator can calculate the flow rate. In preferred embodiments the interference signals are produced using shear plates. In one preferred embodiment useful for monitoring the flow rate of a respirator, the correlation of the fringe intensity values corresponding to the two beam paths is made by a digital computer programmed with an algorithm for making cross correlation analyses that utilizes a fast Fourier transform. The invention is based on measurements of the flow of turbulent inhomogeneities in the fluid flow at two locations with a known separation. In preferred embodiments the turbulent inhomogeneities of the flow are increased by heating the fluid just upstream of the two beam paths. The flow velocity is estimated from the measured travel time, which is required for the flow to move turbulent eddies from one location to another, and the known separation between the two locations. The present invention provides a device that is purely optical and non-invasive and does not possess any significant obstruction to the flow. It creates no significant pressure drop and no pulsed pressure waves in the patient""s airway and can be placed close to the patient, it is not sensitive to gas composition and contamination, it is easy to clean, because it uses a disposable or reusable flow measurement cuvette, and it is more accurate, rugged and reliable than existing sensors on the market.